This invention relates to a seat belt pre-tensioning device, and more particularly to a seat belt pre-tensioning device adapted to tension a seat belt when an accident occurs.
When an accident occurs involving a motor car it is not unusual for the motor car to be decelerated very rapidly, and in some cases the car can be considered to stop virtually instantaneously. When this happens the occupants of the car, who have their own momentum which is related to the speed of the car before the accident, do not stop immediately, but tend to continue to travel forwardly. It is to arrest this forward movement, and to prevent the occupants of the car from colliding with parts of the car that are not stationary, such as the steering wheel, that seat belts are worn.
It is known for seat belts fitted with inertia reels to be worn. The inertia reel tends to reel up any slack in the seat belt when it has been positioned to restrain the occupant of the car, but is designed to "lock" when subjected to a deceleration in excess of a predetermined deceleration. The inertia reel is thus prevented from rotating to pay out the seat belt.
If a car fitted with such inertia seat belts is involved in an accident, when the car is decelerated rapidly the occupants of the car will still tend to move forwards as a consequence of their own personal momentum. During a brief initial time period the seat belt, which is relatively loosely stretched across the occupant of the car, is placed under tension as the occupant moves forwardly relative to the seat. The tension in the belt is applied to the reel of the inertia reel device (which is now "locked") and an extra length of belt is paid out from the reel as the part of the belt that is wound on the reel is tightened. Thus the occupant of the car still moves forwardly. This can occupy the 35 milli-seconds or so after the instant of impact and during this period of time the occupant may move a significant part of the way from the seat towards the steering wheel or dash board. Thus, if the occupant is to be arrested before impact with the steering wheel or dashboard very high retarding forces must be applied to the occupant. These retarding forces must be applied to the occupant of the car in a very short period of time, and thus the retarding forces must be very large if the occupant of the car is to be prevented from impacting with the steering wheel or dashboard. These large forces may thus injur the occupant of the car.
It has been realised that the risk of injuring the occupant of a car with the seat belt if an accident occurs can be reduced if the seat belt arrangement is such that the occupant of the car can be decelerated as soon as possible after the accident occurs. This can be achieved if the occupant is always strapped in tightly, with the seat belt under significant tension at all times. However, it is not practicable to adopt this technique if inertia reels are utilised.